The present invention provides noble metal-containing nanoparticles for producing membrane electrode assemblies (MEAs) for fuel cells, in particular for low temperature fuel cells, for example polymer electrolyte membrane fuel cells (PEM fuel cells) and direct methanol fuel cells (DMFC). New types of colloidal solutions which contain the noble metal alone or in association with other metals are described, wherein the metals are in the form of nanoparticles embedded in a temporary stabilizer. The nanoparticles are used to produce electrocatalysts and catalysed components for fuel cells. Using these nanoparticles, catalyzed ionomer membranes, catalyzed gas diffusion electrodes (so-called “backings”) and membrane electrode assemblies can be produced.
Fuel cells convert a fuel and an oxidizing agent which are spatially separated from each other at two electrodes into electricity, heat and water. Hydrogen or a hydrogen-rich gas may be used as the fuel, and oxygen or air as the oxidizing agent. The process of energy conversion in the fuel cell is characterized by a particularly high efficiency. For this reason, fuel cells in combination with electric motors are becoming more and more important as an alternative to traditional internal combustion engines. The PEM fuel cell is suitable for use as an energy converter in motor vehicles because of its compact structure, its power density and its high efficiency.
The PEM fuel cell consists of a stacked arrangement (“stack”) of membrane electrode assemblies (MEAs), between which are arranged bipolar plates for supplying gas and conducting electricity. A membrane electrode assembly consists of a solid polymer electrolyte membrane, both sides of which are provided with reaction layers which contain catalysts. One of the reaction layers is designed as an anode for the oxidation of hydrogen and the second reaction layer is designed as a cathode for the reduction of oxygen. On these reaction layers are mounted so-called gas distributor structures or gas diffusion layers made of carbon fibre paper, carbon fibre woven fabric or carbon fleece, which facilitate good access by the reaction gases to the electrodes and effective removal of the cell current. The anode and cathode contain so-called electrocatalysts which catalytically support the particular reaction (oxidation of hydrogen at the anode and reduction of oxygen at the cathode). Metals from the platinum group in the periodic system of elements are preferably used as the catalytically active components. In the majority of cases, so-called supported catalysts, in which the catalytically active platinum group metal has been applied in highly dispersed form to the surface of a conductive support material, are used.
The polymer electrolyte membrane consists of proton-conducting polymer materials. These materials are also called ionomers for short in the following. A tetrafluorethylene/fluorovinylether copolymer with acid functions, in particular sulfonic acid groups, is preferably used. Such materials are sold, for example, under the tradenames Nafion® (E.I. DuPont) or Flemion® (Asahi Glass Co.). However, other, in particular fluorine-free, ionomer materials such as sulfonated polyetherketones or polyarylketones or polybenzimidazoles but also ceramic materials can be used.
The performance data for a fuel cell depends critically on the quality of the catalyst layers applied to the polymer electrolyte membrane. These layers usually consist of an ionomer and a finely divided electrocatalyst dispersed therein. Together with the polymer electrolyte membrane, so-called three-phase interfaces are formed in these layers, wherein the ionomer is in direct contact with the electrocatalyst and the gases (hydrogen at the anode, air at the cathode) introduced to the catalyst particles via the pore system.
To prepare the catalyst layers, ionomer, electrocatalyst and optionally other additives are generally blended to form an ink or a paste. To produce the catalyst layer, the ink is applied by brushing, rolling, spraying, doctor blading or printing either to the gas diffusion layer (e.g. carbon fleece or carbon fibre paper) or directly to the polymer membrane, dried and optionally subjected to a secondary treatment. In the case of coating the ionomer membrane with a catalyst layer, the non-catalyzed gas diffusion layers are then mounted on the membrane on the anode and cathode faces and a membrane electrode assembly is then obtained. Alternatively, the catalyst layers may also be applied to the gas diffusion layers. These gas diffusion electrodes (gas diffusion layers plus catalyst layers) are then laid on the two faces of the ionomer membrane and laminated with this, wherein a membrane electrode assembly is also obtained. The prior art in this area is described in patent documents U.S. Pat. Nos. 5,861,222, 5,211,984 and 4,876,115.
The present invention provides noble metal-containing nanoparticles which can be used for the production of catalyzed components and membrane electrode assemblies for low temperature fuel cells (PEMFC, DMFC, AFC or PAFC). The object of the invention are new types of preparations, or colloidal solutions, of noble metal-containing nanoparticles which are embedded in a suitable temporary stabilizer.
Colloidal nanoparticle solutions have been known for a long time. For example, they are used to produce noble metal supported catalysts. Thus, U.S. Pat. No. 3,992,512 describes a process in which colloidal platinum oxide nanoparticles are prepared by decomposing platinum sulfite acid, fixing these to a supporting carbon black and then reducing to platinum. The process is complicated and expensive and provides only electrocatalysts which are contaminated with sulfur due to using sulfur-containing precursor compounds for the platinum. Stabilizers are not used.
DE 197 54 304 A1 describes platinum-containing nanoparticles which are embedded in a polymeric betaine. Polymeric carbobetaine, phosphobetaine and sulfobetaine, which are built up from a non-branched polymethylene main chain and side chains with different types of betaine groups having degrees of polymerization of 50 to 10,000, are described. The method for decomposing these stabilizers is not described. It has been shown that these stabilizers adhere firmly to the noble metal surface, due to their long polymethylene main chains, and thus contaminate the catalytically active catalyst surfaces. For this reason, these nanoparticles are not very suitable as catalytically active species for membrane electrode assemblies in fuel cells. Nothing is reported about the further processing of these in order to produce catalyzed systems (catalysed ionomer membranes, gas diffusion electrodes, etc.).
Furthermore, DE 44 43 705 A1 discloses noble metal colloids which are stabilized with surfactants (such as, for example, fatty alcohol polyglycol ethers or amphiphilic betaines) and can be used for preparing supported electrocatalysts. After attaching these noble metal colloids to the support material, aftertreatment is required in order to remove the surfactants used for stabilizing purposes. During this aftertreatment (generally thermal pyrolysis at temperatures above 400° C.) the colloid particles sinter so that coarse crystallites are produced.
Furthermore, DE 197 45 904 A1 describes a polymer-stabilized metal colloid solution which contains a cation exchange polymer for stabilizing purposes. Here, the noble metal nanoparticles are precipitated in the presence of an ionomer solution (e.g. Nafion®) and isolated as a dry powder. Investigations by the inventors of the present invention have shown that this process does not lead to stable liquid colloid preparations because the ionomer has no surfactant properties and in addition is itself present as particles in the size range 5 to 20 nm (see also X. Cheng et al., J. Power Sources 79 (1999) 75-81). In addition, our work has shown that this process has considerable disadvantages because it provides nanoparticles which are heavily contaminated with foreign ions such as, for example, chloride or sodium. The presence of chloride in particular leads to corrosion and reduced resistance to ageing of the catalyst components prepared using this metal colloid preparation.
Therefore it is the object of the present invention to provide noble metal-containing nanoparticles which form stable solutions over a long time due to the use of a suitable, temporary, stabilizer and contain only marginal amounts of impurities (halogen ions, alkali metal ions, borate, etc.), which are insignificant for use in fuel cells. They are intended to be used directly for catalyzing ionomer membranes and gas diffusion layers for PEM fuel cells, which means that the temporary stabilizer (or protective colloid) has to be completely removable by means of a gentle process without damaging the polymer electrolyte membrane. Furthermore, the nanoparticles are intended to be capable of being prepared in aqueous medium without the addition of organic solvents.